As would be understood by one of skill in the art, a pipette is a device that normally includes a removable pipette tip and is used to transfer or distribute a measured volume of liquid from one location to another. Manually-operated (manual) air-displacement pipettes, which are of the most interest with respect to this application, typically include an elongated hand-holdable pipette body housing an upwardly spring biased plunger unit. The plunger unit includes at least a piston and a plunger rod and is supported for axial movement in the pipette body between a first or upper stop position in which a proximal end portion of the plunger unit extends from a proximal (upper) end of the pipette body, and a second or lower stop position at which all liquid is expelled from the pipette tip. In operation, a pipette user grips the pipette body with a thumb placed over the exposed end (e.g., a plunger button) of the plunger unit. A depression of the plunger unit by the user moves the plunger unit distally (downward) from its upper stop position toward the lower stop position, against the upward bias of a stroke spring.
A plunger unit “home” position is located between the upper stop position and the lower stop position. In known manual pipettes, the home position is defined by a “soft” stop. Such a soft stop is typically created through the use of a second and relatively stiff spring located within the pipette body. This second spring is commonly referred to as a blowout spring, and typically assists in returning the plunger unit toward the upper stop position after a pipette blowout stroke. The blowout spring, as well as the other springs of a typical pipette, are normally compressed by a small amount during installation. As would be well understood by one of skill in the art, this compression imparts a preload to the springs, the associated force of which depends on the spring rate of the given spring and its distance of compression. The home position (soft stop) of the pipette is indicated when a shoulder or some other provided feature of the plunger unit contacts the blowout spring, which is only activated (further compressed) if the depression force exerted on the plunger button exceeds the preload force of the blowout spring.
In the case of a manual pipette with a home position (soft stop), the pipette user can “feel” via the thumb an increased resistance to movement of the plunger unit when the plunger unit reaches the home position. The felt increased resistance to movement is associated with the preload force of the blowout spring, which opposes further downward movement of the plunger unit. Thus, stated another way, the home position of the plunger unit is defined by the point at which the user first feels through the plunger button an increase in the force of resistance to plunger unit movement, without a corresponding further movement of the plunger unit. Continued movement of the plunger unit beyond the home position to the lower stop position defines the aforementioned blowout stroke, and is resisted by a combination of the stroke spring and the blowout spring.
To pipette a liquid using a known manual pipette, a user first depresses the plunger unit as described above, such that the plunger unit is moved away from the upper stop position against the force of the stroke spring. The user halts depression of the plunger unit when the user detects the home position as indicated by the feeling of increased resistance that results from the preload force of the blowout spring and is transmitted to the user's thumb. Once the home position is reached, the user submerges the opening of the pipette tip attached to the pipette in a liquid of interest, and subsequently releases the plunger unit, which is returned to the upper stop position by the stroke spring. The return stroke of the plunger unit and its associated piston creates a vacuum, and an amount of the liquid is consequently drawn into the pipette tip in response.
Because the distance of piston movement from the home position to the upper stop position defines the volume of liquid that will be aspirated into the pipette tip during the aspiration phase of the pipetting operation, it should be understood that pipetting accuracy in the case of known manual pipettes depends greatly on the ability of a user to return the plunger unit to the same home position at the beginning of each pipetting operation. If the user stops the plunger unit short of the home position, a less than intended amount of liquid will be aspirated. If the user drives the plunger unit past the home position, a more than intended amount of liquid will be aspirated. Consequently, an accurate sensing of the home position is critical to obtaining accurate and repeatable pipetting results.
Depressing the plunger unit of a known manual pipette and stopping at the home position is a delicate operation requiring that great care be exercised by the pipette user if accurate and repeatable results are to be obtained. In fact, it has been found that a significant portion of the total time associated with a pipetting operation is occupied by the pipette user manually maintaining the plunger unit at the sensed home position and ready for insertion of the pipette tip into the liquid which is to be aspirated by the pipette.
In practice, it has also been found that most pipette users have great difficulty in consistently stopping the plunger unit of known manual pipettes in the same home position over repeated pipetting operations, and also in maintaining said position until the plunger unit is released to aspirate a liquid. In this regard, one exemplary and fairly common pattern of actual plunger unit depression force and plunger unit travel during operation of a typical and known manual pipette is represented in the depression force versus plunger unit position graph of FIG. 1A. As shown in FIG. 1A, a user initially exerts an actuating force on the plunger unit that causes the plunger unit to move from the upper stop position at point B to the home position at point C (as detected by the user). As explained above, the home position is the position at which the blowout spring is contacted by a shoulder or other feature of the plunger unit and begins to offer an increased resistance to further plunger movement. The home position of the plunger unit is represented in the graph of FIG. 1A by the line extending between point C and point D, which also indicates the increased depression force required to further move the plunger unit at point D as the blowout spring is engaged and begins to compress. The line extending between point D and point E in FIG. 1A represents further movement of the plunger unit from the home position toward the lower stop position during a blowout stroke.
In an ideal depression force versus plunger position graph, the home position of the plunger unit would be represented by a single point (point C in FIG. 1A). In this case, the line between point C and point D would be perfectly vertical, indicating that there is no deviation of the plunger from the home position until a user deliberately depresses the plunger unit with sufficient additional force to begin compression of the blowout spring and the actual blowout stroke (at point D).
In reality, however, the home position as determined by a user over repeated pipetting operations is virtually never a single position. Rather, as illustrated in the enlarged view of FIG. 1B, the home position indicated by the line extending between point C and point D is actually sloped to the right—signifying that an undesired and unintended movement of the plunger unit is possible within the range of actuation force that a user may interpret as being indicative of the home position. This plunger unit movement around the home position is possible because of possible minor misalignment between pipette components, and/or because the home position is indicated by a “soft” stop provided through spring force, and the nature of springs is such that there is a narrow range of depression force that may be exerted between the point of encountering resistance to movement resulting from contact with the blowout spring, and actual compression of the blowout spring (as represented by the line between point C and point D). As a result, a user may exert different amounts of depression force within said range of depression force while still sensing the “home” position, and these different amounts of depression force will result in different positions of the plunger unit and in different aspiration volumes. In this particular example, it can be observed in FIG. 1B that the range of depression force within which a given user may detect the home position is between about 0.75 lbf. and about 2.4 lbf.
While the amount of plunger unit movement per increased magnitude of depression force may be small within the 0.75-2.4 lbf. range, this plunger movement may be nonetheless highly detrimental to producing accurate and repeatable pipetting results. As one example, consider an inexperienced user who is only able to repeat the depression force applied to the plunger unit of the pipette represented in FIGS. 1A-1B within a one pound range (e.g., between 1-2 lbf.). In such a case, it can be observed in FIG. 1B that the home position of the plunger unit will vary by approximately 0.003 inches (i.e., between 0.628 and 0.631 inches). If, for example, the pipette has a 0.620 inch stroke and is set to 100% volume, this 0.003 inch variation in home position represents only a 0.48% variation in aspiration volume. However, if the same pipette is set to a 10% volume setting, the 0.003 inch variation in home position represents a much more significant 4.8% variation in aspiration volume. Therefore, it can be understood how user sensitivity and skill have been paramount to achieving good pipetting results with known manual pipettes.
The above-identified problem may be further exacerbated if the pipette user has what is commonly referred to in the industry as a light or heavy thumb—generally indicating that the user does not have the preferred sensitivity to movement of the pipette plunger unit. Broadly speaking, a user with a light thumb will “feel” the home position at a lower plunger depression force than is actually required to reach the true home position, while a user with a heavy thumb will “feel” the home position at a plunger depression force that is actually greater than the force required to reach the true home position. This results in the two users pipetting different volumes of liquid, neither of which will match the calibrated volume. A user with a light thumb will, on average, pipette less liquid than a user with a heavy thumb.
In addition to the aforementioned problems surrounding plunger movement at the home position, accurate pipetting results also depend on the pipette user matching the same plunger depression force used during calibration of the pipette. Consequently, the ability to repeatably return the plunger unit to as close as possible to the same home position is even more important.
Based on the foregoing description, it should be obvious that there is a need for a manual pipette with which a user is able to more accurately and repeatably detect plunger unit home position within a narrower positional range. Exemplary home position locating mechanism and pipette embodiments described herein satisfy this need.